In recent years, secondary batteries such as lithium ion batteries are receiving increased attention not only as power sources for portable personal computers and cellular phones but also as vehicle power sources for hybrid cars and electric vehicles. In these secondary batteries such as lithium ion batteries, a plurality of electric cells (cells) are connected in series, thereby forming an assembled battery to provide desired output voltage.
In order that an assembled battery properly exhibits its performance, the constituent cells of the assembled battery have to function normally. To this end, the cells are respectively subjected to inspection before they are incorporated as an assembled battery. With a view to ensuring safety required for storage and delivery, the state of charge (SOC) of each inspected cell is adjusted to a value proximate to a minimum applicable percentage by discharging.
FIG. 11 shows a conventional process for manufacturing an assembled battery. In this process, cells are first produced (Step S11). Then, the cells are respectively adjusted to a specified SOC and subjected to inspection (Step S12). The cells, which have passed the inspection, are subsequently discharged so that their SOC becomes a minimum applicable percentage (30% in this process) (Step S13). Thereafter, the cells, which have been discharged, are assembled, thereby forming a cell stack (Step S14). Then, the constituent cells of the cell stack are electrically connected in series thereby to form an assembled battery (Step S16).
There are known other techniques in which the constituent cells of the cell stack are further inspected subsequently to the formation of the cell stack. In the assembled battery manufacturing method disclosed in Patent Document 1 for example, the SOC of each individual cell is firstly adjusted to a value lower than the minimum applicable percentage and then an inspection is conducted on the cell. Thereafter, the inspected cells are assembled and the assembled cells are then charged until their SOC becomes equal to an intermediate applicable value. In this condition, the cells are respectively inspected again. Subsequently, the cells assembled as an assembled battery are subjected to inspection.